fix nits

Author: tesuji

compiler/rustc_mir_transform/src/gvn.rs

@@ -106,12 +106,6 @@ use std::borrow::Cow;
 use crate::ssa::{AssignedValue, SsaLocals};
 use either::Either;
 
-// LLVM optimizes the load of 16 byte as a single `mov`.
-// Bigger values make more `mov` instructions generated.
-// While changing code as this lint suggests, it becomes
-// a single load (`lea`) of an address in `.rodata`.
-const STACK_THRESHOLD: u64 = 16;
-
 pub struct GVN;
 
 impl<'tcx> MirPass<'tcx> for GVN {
@@ -373,6 +367,11 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
     #[instrument(level = "trace", skip(self), ret)]
     fn eval_to_const(&mut self, value: VnIndex) -> Option<OpTy<'tcx>> {
         use Value::*;
+        use abi::HasDataLayout;
+        // LLVM optimizes the load of `sizeof(size_t) * 2` as a single `mov`,
+        // which is cheap. Bigger values make more `mov` instructions generated.
+        // After GVN, it becomes a single load (`lea`) of an address in `.rodata`.
+        let stack_threshold = self.tcx.data_layout().pointer_size * 2;
         let vvalue = self.get(value);
         debug!(?vvalue);
         let op = match *vvalue {
@@ -417,7 +416,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     let ptr_imm = Immediate::new_pointer_with_meta(data, meta, &self.ecx);
                     ImmTy::from_immediate(ptr_imm, ty).into()
                 } else if matches!(kind, AggregateTy::Array) {
-                    if ty.layout.size().bytes() <= STACK_THRESHOLD {
+                    if ty.layout.size() <= stack_threshold {
                         return None;
                     }
                     let mut mplace = None;